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Moon and Spica

StarDate:

May 1, 2015

The brilliant gibbous Moon creeps up on a bright star tonight: Spica, the leading light of the constellation Virgo. The star is directly below the Moon at nightfall. They’ll arc across the southern sky during the night, then set around first light tomorrow.

If you check on them every couple of hours, you might notice that the gap between them is growing smaller during the night. That’s because the Moon moves eastward against the background of stars. It covers a distance equal to its own diameter in a little less than 50 minutes. So between dusk and dawn, the gap between the Moon and Spica will close by a noticeable amount.

At times, the Moon and Spica can draw even closer together. In fact, the Moon sometimes covers up the star, blocking it from view — an event known as an occultation.

Such an alignment is possible because Spica is just a couple of degrees from the Sun’s path across the sky, known as the ecliptic. The Moon stays close to the ecliptic as well, although it moves within a range of a few degrees to either side of that path. So when the geometry is just right, the Moon passes directly in front of the star.

Occultations come in bunches. In fact, there were several over the last couple of years. But that series came to an end late last year, and the next one won’t begin until 2023. Until then, we’ll have to settle for monthly close encounters between the two — like the one that’s in view tonight from dusk ’til dawn.

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